Defense responses to Fusarium oxysporum f. sp. ricini infection in castor (Ricinus communis L.) cultivars

The biophysical and biochemical basis of resistance in castor to wilt caused by Fusarium oxysporum f. sp. ricini was studied. Entry and colonization of fungal mycelium in susceptible (JI-35) and resistant (48-1) cultivars was studied histopathologically by employing bright field, scanning and transmission electron microscopy. Browning of xylem vessels of vascular bundles in susceptible cultivar can be taken as an anatomical parameter for early detection of the wilt disease. Electron microscopy revealed more mycelium and spores in the susceptible cultivar. Cell wall was thick in resistant cultivar than the susceptible cultivar. Biochemical activity of antioxidant enzymes and pathogenesis related (PR) proteins in the roots of resistant and susceptible genotypes of castor were studied at 24, 48, 72 and 96 h after inoculation till 7 days. Studies pertaining to the activity of defense enzymes viz, superoxide dismutase (SOD), peroxidase (POX), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR) and β-1,3-glucanase revealed that the level of defense related enzyme activity of SOD, GR and β-1,3-glucanase was higher in resistant cultivar. However, the activity of CAT reached a maximum at 4 days post inoculation and decreased by 7 days. In contrast, the activity of APX enzyme was higher in susceptible cultivar. Release of these enzymes related to expression of resistant mechanism restricted the browning of xylem vessels in resistant cultivar. The restricted growth of mycelium, absence of browning in xylem vessels and increase in activity of defense related enzymes in resistant cultivar indicates the resistance mechanism in the plant.